Piezoelectric resonators have been used as micro-gravimetric immunoassay devices (See, for example, Joy E. Roederer and Glenn J. Bastiaans, "Microgravimetric Immunoassay with Piezoelectric Crystals", Anal. Chem. 1983, 2333-2336). Changes in the amount of mass attached to the surface cause shifts in the resonant frequency. A device that has an output signal that is significantly affected by the amount of mass attached to one of its surfaces will be referred to herein as a "mass sensor". Selective mass detection is achieved by coating the surface of the piezoelectric crystal with a chemically reactive layer that preferentially reacts with the substance to be detected such that the mass of the chemically reactive layer changes. Such devices function as chemical sensors that can measure the concentration of the selected class of compounds in a solution into which this sensor is immersed. For example, to measure the concentration of specific antibodies in a solution, a sensor is utilized in which the chemically reactive layer contains the antigen(s) corresponding to these antibodies. The concentration of the antibodies in a liquid can be measured by immersing the sensor in the liquid and inferring the change in mass from the change in resonant frequency of the sensor.
The mass sensitivity (i.e., the fractional frequency change divided by the mass change of material deposited on the surface of the sensor) increases as the mass of a bulk wave resonator is decreased or, correspondingly, as the resonator thickness is decreased. A practical lower limit of about 100 microns, corresponding to a resonance frequency of about 20 MHz, is imposed on resonator thickness by manufacturing difficulties. Consequently the sensitivity of a bulk wave resonator sensor is limited.
Surface acoustic wave (SAW) devices have also been used as mass sensors. Mass attached to the surface of the waveguide affects the velocity of wave propagation along the waveguide, thereby producing a phase shift in the output signal from the waveguide. The effective mass per unit length of the waveguide is determined by the penetration depth of the wave in the substrate. In the case of a SAW device, this penetration depth is about one wavelength. As the frequency is increased, the wavelength is decreased and the sensitivity goes up. The concentration of the antigen can be determined from this phase shift.
Bulk wave resonators and waveguides are not particularly sensitive mass sensors because their mass per unit length is relatively large. Surface Acoustic Wave (SAW) devices also make poor chemical sensors in applications which require the immersion of the sensor in a liquid, because the dominant acoustic displacement component couples strongly to compressional waves in the liquid. The reason for this is as follows. The dominant shear vertical component of SAW motion is normal to the surface so that acoustic waves in a SAW device couple to acoustic waves in the liquid. Since the acoustic propagation velocities of Rayleigh waves in solids are almost universally higher than the velocities of compressional waves in liquids, there always exists a direction of radiated acoustic waves in the liquid that, at the surface of the waveguide, are in phase with the Rayleigh mode of the SAW device. Consequently energy will radiate away from the SAW device into the liquid, causing an unacceptable amount of insertion loss.
Lamb Wave devices also make poor chemical sensors for a related reason (See, for example, R. M. White, P. J. Wicher, S. M. Wenzel, and E. T. Zellers, "Plate Mode Ultrasonic Oscillator Sensors", IEEE Trans., vol. UFFC-34, #2, pp. 163.). Lamb wave devices have the same dominant acoustic particle velocity component as SAW devices--i.e., it is also a dominant Shear Vertical wave device and therefore acoustic waves in a Lamb device will couple into acoustic waves in a liquid in which the Lamb Wave device is immersed. However, by decreasing the thickness of the waveguide, the velocity of the mode can be caused to fall below that of the surrounding liquid. This prevents phase matching and hence prevents the radiation of energy into the liquid. Unfortunately, this decreased thickness also causes the velocity of the Lamb Wave to be a strong function of the density of the liquid. This effect masks the mass sensitivity of the sensor. In addition, such a thin waveguide is fragile and is therefore easily damaged.